Computation provides chemical insight into the diverse hydride NMR chemical shifts of [Ru(NHC)4(L)H]0/+ species (NHC = N-heterocyclic carbene; L = vacant, H2, N2, CO, MeCN, O2, P4, SO2, H−, F− and Cl−) and their [Ru(R2PCH2CH2PR2)2(L)H]+ congeners

Abstract

Relativistic density functional theory calculations, both with and without the effects of spin–orbit coupling, have been employed to model hydride NMR chemical shifts for a series of [Ru(NHC)₄(L)H]^0/+ species (NHC = N-heterocyclic carbene; L = vacant, H₂, N₂, CO, MeCN, O₂, P₄, SO₂, H⁻, F⁻ and Cl⁻), as well as selected phosphine analogues [Ru(R₂PCH₂CH₂PR₂)₂(L)H]⁺ (R = ⁱPr, Cy; L = vacant, O₂). Inclusion of spin–orbit coupling provides good agreement with the experimental data. For the NHC systems large variations in hydride chemical shift are shown to arise from the paramagnetic term, with high net shielding (L = vacant, Cl⁻, F⁻) being reinforced by the contribution from spin–orbit coupling. Natural chemical shift analysis highlights the major orbital contributions to the paramagnetic term and rationalizes trends via changes in the energies of the occupied Ru d_π orbitals and the unoccupied σ*_Ru–H orbital. In [Ru(NHC)₄(η²-O₂)H]⁺ a δ-interaction with the O₂ ligand results in a low-lying LUMO of d_π character. As a result this orbital can no longer contribute to the paramagnetic shielding, but instead provides additional deshielding via overlap with the remaining (occupied) d_π orbital under the L_z angular momentum operator. These two effects account for the unusual hydride chemical shift of +4.8 ppm observed experimentally for this species. Calculations reproduce hydride chemical shift data observed for [Ru(ⁱPr₂PCH₂CH₂PⁱPr₂)₂(η²-O₂)H]⁺ (δ = −6.2 ppm) and [Ru(R₂PCH₂CH₂PR₂)₂H]⁺ (ca. −32 ppm, R = ⁱPr, Cy). For the latter, the presence of a weak agostic interaction trans to the hydride ligand is significant, as in its absence (R = Me) calculations predict a chemical shift of −41 ppm, similar to the [Ru(NHC)₄H]⁺ analogues. Depending on the strength of the agostic interaction a variation of up to 18 ppm in hydride chemical shift is possible and this factor (that is not necessarily readily detected experimentally) can aid in the interpretation of hydride chemical shift data for nominally unsaturated hydride-containing species. The synthesis and crystallographic characterization of the BAr^F₄⁻ salts of [Ru(IMe₄)₄(L)H]⁺ (IMe₄ = 1,3,4,5-tetramethylimidazol-2-ylidene; L = P₄, SO₂; Ar^F = 3,5-(CF₃)₂C₆H₃) and [Ru(IMe₄)₄(Cl)H] are also reported.